You need to flood the sphere with value 1 in the paint attributes (make sure you have the solid brush selected), this will set all the fur to length 1. Then leaving the paint fur tool go to the fur attributes and set the length map multiplier to the longest that you want the fur to be.

Now when you go back into the paint fur tool, Maya will see the new map multiplier number as paint attribute value 1 so you can paint the fur shorter by lowering the paint attribute value.

For instance if you have set the length map multiplier to 10 you need to set the paint attribute value to 0.1 to paint fur length 1.

I hope that makes sense,

Mat.